I have a "contraption" where 9 Hoppers (as shown in the Screenshot) next to each other, drop their items into the Hopperline in front of them. 

For example (with just 5 Droppers):

• Dropper1: "Red Wool"
• Dropper2: "Green Wool"
• Dropper3: "Yellow Wool"
• Dropper4: "Red Wool"
• Dropper5: "Purple Wool"
• etc...

So the Hopper Closest to the endpoint in the Line would be the one in front of Dropper1 and the one furthest away is Dropper5.

The order I would expect the items to arrive at the end is:

"Red Wool" -> "Green Wool" -> "Yellow Wool" -> "Red Wool" -> "Purple Wool".

This works if I build it up and try it out. But as soon as I safe and quit, the order is a different one, I couldn't figure out a schema yet. Often it is something like:

"Red Wool" -> "Red Wool" -> "Green Wool" -> "Yellow Wool" -> "Purple Wool"